1. Field of the Invention
The present invention generally relates to a power train, and particularly, it relates to a power train for four-wheel drive vehicles.
2. Description of Relevant Art
There has been disclosed a four-wheel drive vehicle power train in Japanese Patent Application Laid-Open Publication No. 3-224829, and another four-wheel drive vehicle power train in U.S. Pat. No. 5,188,574.
The former power train is shown in FIG. 1, where it is designated by reference character 301. The power train 301 includes a front drive train 309+311+317 and a rear transfer train 323. The front drive train is arranged at an output end of a transmission 305 which is mounted in parallel to a transversely arranged horizontal engine 303 and put in a coaxial relation to left and right front axles. The drive train includes a planetary type final reduction gear 309, a planetary type center differential gear 311 and a front differential gear 317.
Drive torque from the transmission 305 is supplied from an output shaft 307 thereof, via the final reduction gear 309, to a ring gear 313 of the center differential gear 311, where it is shared to be distributed on the one hand from a pinion carrier 315 of the center differential gear 311 through the front differential gear 317 to the front axles, and on the other hand, from a sun gear output shaft 319 via a gear train 321 to the rear transfer train 323, where it is changed in direction to be transmitted via a rear drive train to rear axles.
Drive torque is increased by a speed reduction at the final reduction gear 309, before it enters the center differential gear 311. The reduction gear 309 is indispensable to drive the front axles.
The latter power train is shown in FIG. 2, where it is designated by reference character 405. The power train 405 has a front drive train 409+407+411 and a rear transfer train 413. Drive power from an engine 401 is transmitted via a transmission 403, where it is once changed in speed, to the front drive train, where it is again changed in speed at a final reduction gear 409 before it enters a center differential gear 407 of a planetary type.
In the conventional power trains described, the rear transfer trains are branched from the center differential gears 311, 407 receiving increased torque to drive front axles and have their and associated components increased in size and weight. Such rear transfer trains may be mounted in a vehicle, with their centers of gravity transversely and/or longitudinally offset from a center of gravity of the vehicle, unfavorably displacing this gravity center, causing an undesirable distribution of loads to be imposed on driving road wheels.
The four-wheel drive vehicle power trains may have their components partially employed in two-wheel drive vehicle power trains. However, the existence of power input mechanisms which supply torque to center differential gears 311 and 407 results in the need for significantly different power train configurations for two-wheel drive and four-wheel drive vehicles, with a correspondingly low level of power train component interchangeability.